End-fire synthetic aperture sonar

Abstract

Techniques are provided for implementing an end-fire synthetic aperture sonar system. A methodology implementing the techniques according to an embodiment includes generating a plurality of matched-filtered signals (pings) based on correlations of a transmitted sonar signal with a plurality of reflected or scattered returns of the transmitted signal received from a hydrophone, the reflected or scattered returns associated with a plurality of locations of the hydrophone relative to a location of a transmitter. The method further includes generating a coarse estimate of the locations of the hydrophone based on incoherent cross correlations of the pings, and generating a refined estimate of the locations of the hydrophone based on the coarse estimate and further based on coherent cross correlations of the pings. The method further includes performing delay-and-sum beamforming to combine the pings, the beamforming employing time delays based on the estimated locations of the hydrophone.

1 . An end-fire synthetic aperture sonar system, the system comprising:
 a matched filter circuit to generate a plurality of matched filtered signals (pings) based on correlations of a transmitted sonar signal with a plurality of scattered returns of the transmitted signal received from a hydrophone, the scattered returns associated with a plurality of locations of the hydrophone relative to a location of a transmitter;   a navigation circuit to generate a coarse estimate of the locations of the hydrophone based on incoherent cross correlations of the pings;   the navigation circuit further to generate a refined estimate of the locations of the hydrophone based on the coarse estimate and further based on coherent cross correlations of the pings; and   a delay-and-sum beamforming circuit to combine the pings to generate a beamformed signal, the beamforming employing time delays based on the estimated locations of the hydrophone.   
     
     
         2 . The system of  claim 1 , wherein the coarse estimate generation further comprises:
 sorting the pings based on maximum values of the incoherent cross correlations;   estimating a seafloor range based on the maximum values;   applying time delays to align the pings based on the estimated seafloor range; and   sorting the aligned pings based on estimated distance to the seafloor.   
     
     
         3 . The system of  claim 2 , wherein the refined estimate generation further comprises:
 delaying groups of the coherently cross-correlated pings to a lag number corresponding to a maximum of the coherent cross-correlations;   performing a second coherent cross-correlation between one of the sorted pings that is estimated closest to the seafloor, and a remainder of the sorted pings; and   delaying the coherently cross-correlated pings to a lag number corresponding to a maximum of the second coherent cross-correlation.   
     
     
         4 . The system of  claim 1 , further comprising a beam-steering circuit to apply additional time delays to the pings to steer the beamformed signal in a desired direction. 
     
     
         5 . The system of  claim 1 , further comprising an intensity-envelope calculation circuit to calculate a backscattering strength of the beamformed signal based on attenuation of the beamformed signal in water and range from the estimated locations of the hydrophone to a sediment surface from which the scattered returns are reflected and scattered. 
     
     
         6 . The system of  claim 1 , wherein the transmitted sonar signal is a frequency swept signal ranging from a first frequency to a second frequency. 
     
     
         7 . The system of  claim 6 , further comprising a bandpass filter circuit to filter the plurality of scattered returns of the transmitted signal to a frequency range between the first frequency and the second frequency. 
     
     
         8 . A method for implementing an end-fire synthetic aperture sonar, the method comprising:
 generating, by a processor-based system, a plurality of matched filtered signals (pings) based on correlations of a transmitted sonar signal with a plurality of scattered returns of the transmitted signal received from a hydrophone, the scattered returns associated with a plurality of locations of the hydrophone relative to a location of a transmitter;   generating, by the processor-based system, a coarse estimate of the locations of the hydrophone based on incoherent cross correlations of the pings;   generating, by the processor-based system, a refined estimate of the locations of the hydrophone based on the coarse estimate and further based on coherent cross correlations of the pings; and   performing, by the processor-based system, delay-and-sum beamforming to combine the pings to generate a beamformed signal, the beamforming employing time delays based on the estimated locations of the hydrophone.   
     
     
         9 . The method of  claim 8 , wherein the generating of the coarse estimate further comprises:
 sorting the pings based on maximum values of the incoherent cross correlations;   estimating a seafloor range based on the maximum values;   applying time delays to align the pings based on the estimated seafloor range; and   sorting the aligned pings based on estimated distance to the seafloor.   
     
     
         10 . The method of  claim 9 , wherein the generating of the refined estimate further comprises:
 delaying groups of the coherently cross-correlated pings to a lag number corresponding to a maximum of the coherent cross-correlations;   performing a second coherent cross-correlation between one of the sorted pings that is estimated closest to the seafloor, and a remainder of the sorted pings; and   delaying the coherently cross-correlated pings to a lag number corresponding to a maximum of the second coherent cross-correlation.   
     
     
         11 . The method of  claim 8 , further comprising applying additional time delays to the pings to steer the beamformed signal in a desired direction. 
     
     
         12 . The method of  claim 8 , further comprising calculating a backscattering strength of the beamformed signal based on attenuation of the beamformed signal in water and range from the estimated locations of the hydrophone to a sediment surface from which the scattered returns are reflected and scattered. 
     
     
         13 . The method of  claim 8 , wherein the transmitted sonar signal is a frequency swept signal ranging from a first frequency to a second frequency. 
     
     
         14 . The method of  claim 13 , further comprising bandpass filtering the plurality of scattered returns of the transmitted signal to a frequency range between the first frequency and the second frequency. 
     
     
         15 . At least one non-transitory computer readable storage medium having instructions encoded thereon that, when executed by one or more processors, cause a process to be carried out for implementing an end-fire synthetic aperture sonar, the process comprising:
 generating a plurality of matched filtered signals (pings) based on correlations of a transmitted sonar signal with a plurality of scattered returns of the transmitted signal received from a hydrophone, the scattered returns associated with a plurality of locations of the hydrophone relative to a location of a transmitter;   generating a coarse estimate of the locations of the hydrophone based on incoherent cross correlations of the pings;   generating a refined estimate of the locations of the hydrophone based on the coarse estimate and further based on coherent cross correlations of the pings; and   performing delay-and-sum beamforming to combine the pings to generate a beamformed signal, the beamforming employing time delays based on the estimated locations of the hydrophone.   
     
     
         16 . The computer readable storage medium of  claim 15 , wherein the process further comprises:
 sorting the pings based on maximum values of the incoherent cross correlations;   estimating a seafloor range based on the maximum values;   applying time delays to align the pings based on the estimated seafloor range; and   sorting the aligned pings based on estimated distance to the seafloor.   
     
     
         17 . The computer readable storage medium of  claim 16 , wherein the process of generating the refined estimate further comprises:
 delaying groups of the coherently cross-correlated pings to a lag number corresponding to a maximum of the coherent cross-correlations;   performing a second coherent cross-correlation between one of the sorted pings that is estimated closest to the seafloor, and a remainder of the sorted pings; and   delaying the coherently cross-correlated pings to a lag number corresponding to a maximum of the second coherent cross-correlation.   
     
     
         18 . The computer readable storage medium of  claim 15 , the process further comprising applying additional time delays to the pings to steer the beamformed signal in a desired direction. 
     
     
         19 . The computer readable storage medium of  claim 15 , the process further comprising calculating a backscattering strength of the beamformed signal based on attenuation of the beamformed signal in water and range from the estimated locations of the hydrophone to a sediment surface from which the scattered returns are reflected and scattered. 
     
     
         20 . The computer readable storage medium of  claim 15 , wherein the transmitted sonar signal is a frequency swept signal ranging from a first frequency to a second frequency, and the process further comprises bandpass filtering the plurality of scattered returns of the transmitted signal to a frequency range between the first frequency and the second frequency.